The machines used in mechanical tree harvesting inevitably cause a certain loading on the floor of the forest, the ground, and, on thinning sites, also on the root systems of the trees left to grow. The problem is typically less in the case of a harvester, which does not need to carry a load in addition to its own weight, as a forwarder does. In other words, there is usually a significant difference in the surface pressure imposed by the machines on the ground. The surface pressure imposed on the ground by the machines has indeed been begun to the reduced by various known solutions, such as by using, in place of a conventional axle, a so-called set of swing-bogie axles, in which there are four wheels in place of a conventional axle, which are able to distribute the surface pressure over a wider area against the surface of the ground. The surface pressure has been reduced by using, for example, a crawler track made from steel or rubber, manufactured and developed for the purpose and running around the wheels of the swing bogie. Steel crawler tracks, surfaced with rubber, and other constructions based on the aforementioned materials, are also known from the prior art.
From the point of view of timber harvesting, the freezing of the ground in winter also has a great effect on the damage to the ground caused by mechanical timber harvesting. On frozen ground, timber harvesting can usually be performed using a forest tractor equipped with a traditional chassis construction, without significant ground damage. But if the winter is mild, the ground may not freeze properly, so that a forest tractor equipped with wheels will leave deep grooves after it, which the entity responsible for the timber harvesting may have to make good in summer. Due to this, an increasingly small surface pressure is required of forest tractors and forest machines in general, which reduces the creation of ground damage occurring in timber harvesting taking place in mild winters. In addition to ground damage, a problem may simply be that the ability of the forest machine to move forward is insufficient when the surface pressure imposed on the ground becomes too great, so that the forest machine sinks partly into the ground, or becomes stuck in place.
As is known, the surface pressure can be reduced by means of various bogie structures to be installed in connection with a swing bogie, in which case, in terms of operating principle, special crawler-track units are approached, though in these solutions obvious defects and/or problems can be detected. For example, in the case of a steel crawler track fitted in connection with a swing bogie, the transmission of power from the bogie's driving wheels is based on friction, and not on the shape-locking between toothing in the crawler track and a special drive wheel as in most actual crawler-track units and crawler-track vehicles. This means that the steel crawler track must be tensioned considerably tightly, if it is wished to avoid the danger of the track slipping. This tensioning then stresses the swing bogie unnecessarily, and particularly the structures of its bogie casing and the wheel hubs—while nevertheless being unable to ensure the reliable and certain transmission of power between the pneumatic tyres and the bogie. Such a bogie structure is also liable to pieces of wood and, for example, hard-packed snow, penetrating between the bogie and the wheels, which may cause even great stresses, particularly in the structures of the swing bogie casing and the wheel hubs. Similar bogies are also known in the form of various composite structures, such as rubber/steel composite structures.
On the other hand, the use of a pneumatic tyre, which is expensive in manufacturing costs, complicated in construction, and liable to puncturing, is, as such, questionable inside the bogie structures described above, due to which the pneumatic tyres could advantageously be replaced by some simpler, cheaper, and more durable construction. In addition, particularly in load-carrying forest machines, such as forwarders, the air pressure in the pneumatic tyres must be set to be very high, for example, 4-6 bar, on account of the load-bearing capacity and technical durability of the tyre, so that in practice the flexibility of the tyres is already very poor and the use of pneumatic tyres in the application is, as such, questionable. Further, a problem with the construction combining a swing bogie equipped, for example, with pneumatic tyres, and a crawler-track running around the pairs of wheels, is that the swing bogie does not distribute the surface pressure acting on the ground on a sufficiently large surface area. In other words, the surface pressure is higher precisely under the wheels of the pairs of wheels, the area between them being subject to a lighter loading and thus a lower surface pressure.
Finnish patent FI 121991 is known from the prior art and discloses a modular crawler-track unit, which can be used in existing forest machines in place of sets of bogie wheels, in order to reduce surface pressure. This crawler-track unit is attached to the forest machine in connection with a wheel hub, so that the shaft coming from the drive/differential wheels rotates the drive wheel of the crawler-track unit. In this crawler-track unit, three crawler wheels are used, which form a triangular endless circulation for the crawler track. However, there are more small support wheels against the ground, but these do not conform to the ground. The drive wheel of the crawler-track unit is also coaxial with the drive shaft. In order to achieve a sufficient grip for the drive wheel, the drive wheel should be covered by the crawler track over a sufficiently large area. Because the crawler-track unit is pivoted at its centre to the chassis of the forest machine, the crawler-track unit must be high. When being driven, the height of the crawler-track unit causes it to tend to rise vertically when crossing obstacles. This means that the pivoting movement of the crawler-track unit is very limited, attenuated, or even prevented. The rigid crawler-track unit is, in turn, very stiff on the ground and cannot conform to unevenness in the ground. In addition to the aforementioned problems, the construction has the problem that, due to the geometry of the circulation of the crawler track, it has disadvantageous angles of incidence and lag, which reduces the ability of the forest machine to progress over difficult ground, especially in deep snow, or deep soft ground, as well as when encountering sudden steep-edged obstacles such as stones or tree-stumps.